1. Field of the Invention
The present invention relates to a suspension for a vehicle such as an automobile, and more particularly, to an active suspension which incorporates servo actuators for supporting the vehicle body from the vehicle wheels.
2. Description of the Prior Art
In the article "Suspension without springs" in the magazine "Motor" published in Great Britain on 10 September 1983, in the article "On active service" in the magazine "Autocar" published in Great Britain on 10 September 1983, and in Japanese Patent Laying-open Publication 60-500662 (1985), there is disclosed a suspension having a plurality of electro-hydraulic type servo actuators provided corresponding to each vehicle wheel and producing a support force to support the vehicle body with respect to the corresponding vehicle wheel, a movement detection means detecting the amount of relative movement between the vehicle wheel and the vehicle body, and a control means controlling an increase or decrease in the support force of each servo actuator based on a signal from the movement detection means.
In the above described suspension, when there is a relative movement between the vehicle wheel and the vehicle body caused by irregularities in the road surface, each actuator produces a support force corresponding to the relative amount of movement, and therefore, as the vehicle incorporating this suspension travels over an irregular road surface, the support force of each actuator with respect to the vehicle body is increased and decreased. This increasing or decreasing force is transmitted to the vehicle body, and as a result, the vehicle body will oscillate vertically in the same way as in a vehicle incorporating a conventional spring type of suspension. In other words the suspension described in the above bibliography, in spite of incorporation of the servo actuators, behaves like a conventional suspension based upon the springs and the shock absorbers, and therefore, this suspension does not prevent vertical oscillation of the vehicle body when the vehicle is traveling over an irregular road surface or the like.
In view of the above described problem in the suspension described in the above bibliography, as a new active suspension constructed so that when a vehicle is traveling over a road surface with irregularities within a stroke limit of the actuators there is absolutely no vertical oscillation of the vehicle, and when the vehicle is traveling over a road surface such as a hill or with large undulations, the vehicle moves smoothly with the road surface, so that the ride comfort of the vehicle can be substantially improved as compared with a conventional spring type suspension or the suspension described in the above bibliography, the present inventor has proposed, in Japanese Patent Application 61-180942 (1986) filed on 31 July 1986 and laid open to public on 18 February 1988, an active suspension for a vehicle comprising: a plurality of cylinder-piston type actuators provided corresponding to each vehicle wheel and producing a supporting force supporting said vehicle body with respect to the corresponding vehicle wheel; a movement detection means detecting the amount of relative movement between said vehicle wheel and said vehicle body; a means for determining the rate of relative movement between said vehicle wheel and said vehicle body; and a calculation control unit which receives from said movement detection means and said means for determining the rate of relative movement respectively a signal indicating the amount of relative movement between said vehicle wheel and said vehicle body and a signal indicating the rate of relative movement between said vehicle wheel and said vehicle body, determines based on these signals the vertical displacement of the road surface, controls, when the vertical displacement of said road surface is within the stroke limit of said actuator, said actuator to produce a support force sufficient to support the appropriate load component of said vehicle body, and calculates, when the vertical displacement of said road surface is in excess of the stroke limit of said actuator, the value corresponding to the support force of said actuator necessary to travel along said road surface based on said decision result, and controls said actuator based on this calculation result and increases or decreases the force acting between said vehicle wheel and said vehicle body through the actuator, and has further proposed a similar active suspension for a vehicle in which additionally a comparison is made of the actual support force of said actuator detected by a load detection means and the calculated load force from said calculation control means, and a feedback control is carried out in order that the difference between these two is brought to zero.
According to the active suspension of the above proposal, when the vertical variation in the road surface is within the stroke limit of the actuator, then each actuator is controlled so as to produce a support force sufficient to support the appropriate component of the weight of the vehicle body, and as a result the vehicle body is supported by a constant force from the actuators, and the vertical displacement of the vehicle wheels following the road surface is not transmitted to the vehicle body and therefore when the vehicle is traveling over a level road surface or a road surface with small undulations, the drive attitude of the vehicle (for example horizontal motion in the case of driving horizontally) is maintained by inertia, and there is absolutely no vertical oscillation.
When the vertical variation in the road surface exceeds the stroke limit of the actuators, based on the determination result of the vertical variation in the road surface a value corresponding to the support force necessary for each actuator for driving over this road surface is computed, the actuators are controlled based on this computation result, and the force acting between the vehicle wheel and the vehicle body through the actuator is increased or decreased, and as a result, when the vehicle is traveling over a road surface with large undulations or a hill, the vehicle body moves vertically corresponding to the vertical displacement of the road surface in such a way that the piston of the actuator does not move relative to the cylinder to exceed the stroke limit, and thereby the vehicle body is also caused to move smoothly along the road surface.
However, in an active suspension according to the above proposal, in the case that as in that embodiment the actuators are controlled based on the relative displacement between the vehicle wheels and the vehicle body and the average value of the relative displacement between the rear wheels and the vehicle body, then the vehicle can be caused to run along the road surface even on inclines and large unevennesses, but it is not possible to make the vehicle body travel parallel to the road surface. For example, when the vehicle is running along a horizontal road, and begins to climb a hill, the vehicle body will begin to climb the hill in a horizontal state, and may give the occupants of the vehicle an uncomfortable feeling.
Furthermore, in the case that the active suspension of the above described prior proposal is provided on each vehicle wheel, the vehicle body can be caused to travel parallel to the road surface, but since the actuators are controlled mutually independently, it is difficult to provide a balanced control over all the vehicle wheels, and since the control results of the actuator for one vehicle wheel have an effect on the motion of portions corresponding to other vehicle wheels, then there is the problem that the vehicle body attitude may easily become unstable.
The present invention considers the above described problems in the active suspension of the above described prior proposal, and has as its object the provision of an improved active suspension such that not only is it possible to cause the vehicle to travel along the road surface even in the case of an incline or large unevenness, but also to ensure that the vehicle body moves parallel to the road surface in a stable attitude.